Grimhilde's Cauldron is an enchanted, talking cauldron belonging to Grimhilde.
The Enchanted Cauldron was a cauldron which could perform magical spells. It could conjure images, brew potions, summon things, and, on Halloween, would grant its owner one wish. People could be magically imprisoned within its depths, and at some point before 2005, a number of villains from all over space and time had suffered this fate, though this did not last.
It is unknown how the cauldron came to be alive (it could be inhabited by a spirit much like the Black Cauldron, it could be due to the potions brewed in it over the years, or it could be an effect of having the souls of multiple villains trapped in its depths), but it dates back to the Dark Ages, when it had ended up in the collection of the Witches of Morva. When they destroyed their cottage to unearth the Black Cauldron for Taran, the Witches enchanted all their cauldrons (magical and otherwise) to make them fly away and clear the path; the Cauldron was one of the pots sent adrift that day.
The Cauldron ended up in the possession of Grimhilde, who used it for the brewing of potions and casting of spells when she was still queen. It was kept in the dungeons of her castle, along with her Magic Mirror and other magical items. In 1937, she used the cauldron to brew a potion to turn herself into a hag, and again to create the "Sleeping Death" potion. She was defeated by the Seven Dwarfs soon after this, and the cauldron presumably sat unused until 2005, when Grimhilde returned to her castle for revenge. She called upon the cauldron to show her many villains from across time and space, so that she could decide which to summon to help her in her revenge.
After seeing many of the villains, she decided to summon them all to form an army. The cauldron warned her that no good could come of it, showing her the defeat of the Horned King, another villain who had attempted to summon an army from a cauldron. Ignoring the cauldron's warnings, Grimhilde wished that it would join her with all of the villains it had shown her. However, since many of these villains had died, Grimhilde, upon being joined with them, was condemned to the depths of the cauldron along with them. She presumably escaped in some way, along with many of the rest of villains, as that was not the last seen of them. The cauldron is most likely still in the castle dungeons, unless it was destroyed during the unknown event that allowed Grimhilde to escape it's confines.